Ординатура / Офтальмология / Английские материалы / Diagnosing and Treating Computer-Related Visual Problems_Sheedy, Shaw-McMinn_2003
.pdf124 Diagnosing and Treating Computer-Related Vision Problems
Corneal Dystrophies and Degeneration
Loss of transparency of the cornea results in light scatter and loss of contrast, which can make computer displays difficult to see. Antireflection coatings on their spectacles enhance contrast and improve vision. Often, patients with corneal dystrophies and degeneration respond well to tints blocking the short wavelengths of light, such as pink or rose tints. Specular reflections and office lighting can worsen the effects of corneal conditions.
Summary
Dry eye, ocular and systemic conditions, and computer vision symptoms can be exacerbated by environmental conditions present in the workstation. Lighting, reflections, computer displays, workstation ergonomics, and air quality can contribute to a worsening of symptoms. The following four chapters deal with environmental factors contributing to computer vision discomfort.
Action Items
1.Decide on a treatment regimen to alleviate dry eye symptoms.
2.Train staff to answer questions concerning dry eye and educate the computer user about contributing factors.
3.Obtain artificial tear samples and promotional materials.
4.Decide on which low-water-content contact lenses you will prescribe for contact-lens patients with dry eye.
5.Train staff to explain how ocular diseases can interfere with computer use.
6.Develop protocol for demonstrating antireflective coatings, tints, and Corning photochromic filter lenses to these patients.
7.Train your staff to explain workstation factors that may contribute to dry eye.
8.Create a handout to give to patients to self-evaluate their workstations.
Treating Ocular Pathologic Conditions That Affect Computer Vision 125
References
AOA Optometric Clinical Practice Guideline: Care of the Contact Lens Patient. St. Louis: American Optometric Association, 2000.
Eng WG. Survey on eye comfort in aircraft: flight attendants. Aviat Space Environ Med 1979;50(4):401–404.
Franck C. Eye symptoms and signs in buildings with indoor climate problems. Acta Ophthalmol (Copenh) 1986;64:306–311.
Bartlett JD, Jaanus SD. Clinical Ocular Pharmacology, 3rd ed. Boston: Butterworth– Heinemann, 1995;256.
Karpecki PM, Thimons JJ. Dry eye management for the new century. Rev Optom 2001;138(2):64–72.
Lemp MA. Epidemiology and classification of dry eye. Adv Exp Med Biol 1998;438: 791–803.
Morris S. Postsurgical dry eye. Optom Manage May 2000;109–112.
Patel S, Henderson R, Bradley L, et al. Effect of visual display unit use on blink rate and tear stability. Optom Vis Sci 1991;68(11):888–892.
Tsubota K, Nakamori K. Dry eyes and video display terminals. Letter to editor. N Engl J Med 1993;328:524.
Yaginuma Y, Yamada H, Nagai H. Study of the relationship between lacrimation and blink in VDT work. Ergonomics 1990;33(6):799–809.
8
Lighting
There are several aspects of working at a computer that make it especially visually demanding or contribute directly to symptoms for computer users, or both. Mitigation of these problems can often directly eliminate symptoms or reduce the demands of the task so that symptoms don’t result from marginal visual disorders. The most important visual environmental factors are listed in Box 8-1. They are presented in detail in this chapter, as well as Chapters 9–11.
General
Improper lighting is likely the largest environmental factor contributing to visual discomfort. Good lighting is actually quite easy to understand, but most people have a misconception concerning what is important about lighting. The common thought is that good lighting is evaluated on the basis of how much light is present, or the footcandles (fc) of illumination. Although it is necessary and important to have adequate illumination, this is not usually the most important aspect of good lighting. In fact, there is often too much light in the office environment. The most important aspect of lighting is the distribution of light in the room.
The eye care practitioner’s responsibility is to educate the patient on the effect of poor lighting on performance at the computer and to advise ways of improving the patient’s situation. The eye care practitioner who offers site-visit evaluation of working environments can do lighting measurements and give proper recommendations.
127
128 Diagnosing and Treating Computer-Related Vision Problems
Box 8-1
Aspects of the computer work environment that contribute to eye problems
•Lighting geometry and quantity
•Glare from windows or overhead lights
•Screen reflections
•Computer display design (contrast polarity, resolution, flicker, etc.)
•Workstation arrangement
•Office air quality
Making Light Measurements
Before having a discussion on light analysis, a few words on light measurements, the tools required for light measurements, and how to use the tools are required. There are two types of light measures that are critical in evaluating an office environment: illumination and luminance. Measuring these aspects of lighting can be very useful when performing on-site evaluations of a computer workplace (see Chapter 12).
Illumination is a measure of the amount of light (lumens) falling on a surface. The common units of illumination are fc (1 lumen/ft2 of surface) and lux (1 lumen/m2 of surface). There are 10.76 ft2 contained within 1 m2; therefore, 10.76 lux is the same amount of illumination as 1 fc (i.e., the same light density per area). Lighting engineers and architects usually design the lighting in a room to provide a predetermined illumination level. An illumination meter is often simply called a light meter (Figure 8-1). The sensing device is simply placed at the location at which a measure of illumination is desired. Sometimes the sensing device is an integral part of the unit and sometimes extends from the unit with a wire attachment. Take care not to cast shadows on the measurement device while making a measurement. The measure of illumination depends on the height in the room—for example, it is typically
Lighting 129
FIGURE 8-1. Example of a light meter that measures illumination.
greater at desk level than on the floor because of distance to ceiling lights. Normally, the illumination is measured on horizontal surfaces; therefore, the measuring device should be oriented horizontally. However, if you desire to measure illumination on the surface of the computer display, then the measuring device should be oriented parallel to the display surface. Illumination can vary quite widely in various office locations; therefore, it is important to measure it at the most important user locations and to specify where each measurement is made. For example, “General room illumination at desk level was 50 fc; at desks by the windows, it was 120 fc; and for those employees using a desk lamp, the reference materials had 90 fc of illumination.”
Luminance is the other important measure of light. Luminance is the most important attribute insofar as vision is concerned, because it is a measure of the amount of light coming toward the eye from an object (per angular area of the object). Quite simply, we judge the brightness of objects based on their luminance. Brighter objects have higher luminance. The relationship between luminance and brightness is not
130 Diagnosing and Treating Computer-Related Vision Problems
FIGURE 8-2. Example of a light meter that measures luminance.
linear—that is, each subsequent “step” in perceptual brightness requires a greater amount of luminance addition than did the previous step. This is similar to most other human senses such as sound and touch. The result of this nonlinear scaling is that our visual system is good at determining whether one object is brighter or dimmer than another, but our visual system is not good at judging the amount of luminance. This amount must be measured with a luminance meter or photometer (Figure 8-2). The common unit of luminance is candle/m2 (cd/m2).
The luminance of an object is measured by viewing it through a luminance meter. The viewer usually sees a circular reticle; the meter should be rotated so that this “measuring circle” is located on the object for which the luminance measurement is desired. (Most 35mm cameras effectively measure luminance—that is, the user views the scene through the camera. The light needle moves or the camera settings change depending on the luminance of the scene.) For a valid measurement, the entire measuring circle should be filled with the object being measured. The same object can have a different luminance depending on the angle from which it is viewed. For example, the luminance (or brightness) of metal or glossy paper sitting on a
Lighting 131
table can change quite significantly if viewed from a different angle. Therefore, in computer-using environments, it is usually most important to make measurements from the normal location of the eyes of the computer user. Normally it is best to sit in the chair of the computer user to make the luminance measurements. A typical set of luminance measurements for a computer user might be
Computer display = 80 cd/m2
Reference documents = 150 cd/m2 at 30 degrees to the left Immediate background of display under shelving = 15 cd/m2 Open window in afternoon = 2,000 cd/m2 at 50 degrees to the right Overhead fluorescent lights = 3,000 cd/m2
Insofar as the visual system is concerned, luminance distribution in the field of view is the most important aspect of lighting. The geometry of the lighting in the room is normally the most important aspect of lighting as it relates to visual comfort. The geometry of the lighting is akin to the quality of the lighting. This involves not only the light sources, but also how the light is directed into the office through the use of lighting fixtures, baffles, blinds, drapes, and others and how it is reflected from the various surfaces in the room such as walls, ceilings, and furniture. A good lighting situation is one in which all of the visual objects in the field of view have nearly equal brightness (i.e., they all are similar in luminance). The correlate is that bad lighting is a situation in which objects in the field of view have large differences in luminance.
The most important principle of good lighting is to eliminate bright sources of light from the field of view and to obtain a relatively even distribution of luminance (brightness) in the field of view.
Glare Discomfort
Bright sources of light in the field of view create glare discomfort. Although this is a well-known phenomenon and the threshold sizes
132 Diagnosing and Treating Computer-Related Vision Problems
and locations of visual stimuli, which cause glare discomfort have been determined (Guth, 1981), the physiologic basis for glare discomfort is not known. It may be related to pupillary fluctuations. Because large brightness, or luminance, disparities in the field of view can cause glare discomfort, it is best to have a visual environment in which the luminances are relatively equal.
Primarily because of glare discomfort, the Illuminating Engineering Society (IES) has established and the American National Standards Institute (ANSI) has accepted (ANSI/IESNA, RP-1-1993) certain maximum luminance ratios that should not be exceeded. The luminance ratio should not exceed 1 to 3 or 3 to 1 between the task and the immediate visual surroundings (approximately 25 degrees), nor should the ratio exceed 1 to 10 or 10 to 1 between the task and more remote visual surroundings (ANSI/IESNA, RP-1-1993). A person is at greater risk for experiencing glare discomfort when the source has a higher luminance and when the source is closer to the fixation point (Box 8-2).
Box 8-2
Display and glare source luminance
|
Luminance |
Visual object |
(candle/m2) |
Dark background display |
20–25 |
Light background display |
80–120 |
Reference material with 75 footcandles |
200 |
Reference material with auxiliary light |
400 |
Blue sky (window) |
2,500 |
Concrete in sun (window) |
6,000–12,000 |
Fluorescent lights (poor design) |
1,000–5,000 |
Auxiliary lamp (direct) |
1,500–10,000 |
|
|
Lighting 133
There are many objects in the field of view that can cause luminance ratios in excess of those recommended by ANSI/IES. Box 8-2 lists the typical luminance of objects in the field of view. The luminance of the screen should be noted first, because it is the screen that the person is viewing. This usually sets the lower end of the luminance ratio. The luminance values in Box 8-2 readily show that luminance ratios can greatly exceed those recommended by ANSI/IES.
The glare sources are a particular problem for computer workers who are generally gazing horizontally in the room. This results in light fixtures and other glare sources being closer to the fixation point for today’s computer worker compared to yesterday’s office worker who looked down at the desk. Bright open windows pose the same risk as
overhead light fixtures. Workers are also at greater risk for discomfort glare if they use a dark background computer display, resulting in greater luminance disparity between the task and other objects in the room. Other sources of large luminance at the computer workstation include white paper on the desk, white desktop surfaces, and desk lamps aimed toward the eyes or that illuminate the desk area too highly.
A common source of discomfort glare is shown to the left. Light often leaves the overhead fluorescent fixture in a wide angle, resulting in light directly entering the eyes of the workers. It is very common for the luminance of the fixture to be more than 100 times greater than that of the video display that the worker is viewing, far exceeding
the IES recommended maximum.
Good lighting design can significantly reduce discomfort glare. Light leaving the fixture can be directed so that it goes straight down and not into the eyes of the room occupants. This is most commonly accomplished with the louvers in the luminaire or fixture. However, this results in severe shadows, and the glare is still there; it is just more peripheral in the field of view. An even better solution is indirect lighting in which the light is bounced off the ceiling, resulting in a large low-luminance source of light for the room.